Numerous areas, both in science and industry level, have benefited in recent years from the application of ultrafast mode-locked lasers. The introduction of a saturable absorber (SA) in the cavity is a convenient and simple way to achieve passive self-starting mode-locking in fibre lasers. The applicability of such passively mode-locked ultrafast fibre sources is mainly limited by the achievable energy and peak power, the latter usually going up to only a few kW in the femtosecond pulse range. In our current study we rely on a novel InN-based SA, recently presented and studied in [1,2], to set-up an ultrafast, high peak power ultrafast passively mode-locked fibre laser with operation wavelength at 1.56 μm in telecommunication C-band.
The fibre laser is implemented as a ring resonator. It is based on a commercial EDFA as gain medium and the aforementioned SA [1], that consists of InN grown by molecular beam epitaxy on a GaN-on-sapphire template, used in reflection configuration as SESAM, which is located in free space and connected to the ring through an optical circulator. The outstanding properties of this material are here further validated, especially its tolerance to extremely high fluences (>1TW/cm2). The cavity length is increased introducing a 1-km standard single mode fibre (SSMF).
Note that no polarization control is needed in the system and, as shown in [2], the basic ring configuration around 40-m-long can achieve stable mode-locking with sub-250 fs pulses and a repetition rate around 5 MHz. However, if polarization control is introduced, it is possible to tune the temporal and spectral widths of the pulses, from 150 fs to 320 fs.
With a 1 km-long cavity, fundamental mode-locking is easily achievable increasing cavity losses with a VOA. Since symmetric Gaussian pulse spectra are generated, very similar to those delivered in the basic ring configuration, it is possible to assume that all the energy is contained in the pulse spectrum, without significant contribution of continuous white noise. This results in pulses with a temporal width at FWHM of 239 fs and a spectral width of 25.4 nm. Since the repetition rate is 196.4 kHz and the average power is 30.5 mW, pulses with peak power of 650.5 kW and pulse energy of 155.3 nJ are produced.
If losses in the cavity are gradually reduced, mode-locking to higher harmonics is obtained in cavities up to 6-km-long, proving the excellent stability of the system. This allows a broad choice of high peak powers and repetition rates.
To summarise, for the first time to our knowledge, it has been experimentally demonstrated a polarization-independent passive ultrafast harmonic mode-locked ring fibre laser operating at 1.56 μm achieved with a simple low-cost set-up and the use of an InN-based SESAM, which in combination with a 1-km-long SSMF cavity, delivers femtosecond pulses with a peak power of 650.5 kW, the highest obtained to date, to our knowledge, in this kind of lasers.
References
[1] F. B. Naranjo et al., Appl. Phys. Lett., Vol. 98, no. 3, p. 031902 (2011).
[2] M. Jimenez-Rodriguez et al., Opt. Express, Vol. 25, no. 5, p. 5366 (2017).

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